This invention pertains to the art of means for varying the impedance through a circuit and more particularly to potentiometers, rheostats, and the like.
The invention is particularly applicable to a hand actuated rheostat and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications and may be advantageously employed in other electrical environments and applications.
Typically, a variable impedance element employs a movable wiper designed for electrical contact with a circuit element that may comprise conductive and/or resistive elements. The wiper is designed for sliding contact with the conductive and/or resistive elements and a great deal of effort has been expended to eliminate problems associated with mechanical wear resulting from the abrasive, wiping action. Although advancements have been realized in the art, the wear and degradation of the mechanically sliding elements continues to be a persistent problem.
Many of the variable impedance devices today employ a substrate in which resistive or conductive elements or tracks are traced thereon. Generally, these elements or tracks are printed on the substrate and are referred to as polymer thick film elements. These elements are not very abrasion resistant and usually require low contact forces with an associated wiper to eliminate harmful wearing action. The wiper may be electropolished to insure a burr-free surface which limits some of the abrasive interaction, but this is still not deemed to be an ideal solution.
Yet another problem associated with the polymer thick film elements is the resultant dead spots that may be formed therein. The films are formed of a polymeric material with carbon particles mixed in. If the resistivity of the element is high, there is, in turn, a low content of carbon. The uneven distribution and resultant absence of carbon in selected locations may cause dead spots in the resistive elements. To overcome this problem, past practice utilized multi-point wipers in an effort to avoid the dead spots and assure electrical contact with the carbon particles. A multi-point wiper normally requires a delicate, costly stamping operation. Additionally, conventional metallic wipers must be made of corrosion-resistant, conductive materials or must be plated adding even greater expense to an already expensive component.
One attempt to overcome some of these wear problems is found in U.S. Pat. No. 3,123,793, issued Mar. 3, 1964 to Flygstad, et al. That patent particularly discloses an impedance element having radially extending terminals at each end that extend outwardly from a generally cylindrical housing. A closed-loop, conductive tape contacts selected portions of the impedance element as a result of rotation of an elastic member. A central rotor member receives the elastic member in a drive groove so that rotation of the rotor is imparted to the elastic member which, in turn, abuts the conductive tape to provide selective electrical contact between various arcuate portions of the conductive tape and the impedance element. Although this patent has met with success, it is not believed to adequately solve all of the above detailed problems in an economical manner.
The subject invention is deemed to overcome these problems and others in an economical, reliable manner.